The present invention relates to machine vision systems and more specifically to systems for adjusting the focus of a liquid or micro-fluidic lens used in a machine vision system.
Machine vision systems have been developed for many different applications. For example, vision systems have been developed for reading bar codes and other types of symbols placed on packages or products to obtain information there from. Other vision systems have been developed for inspecting manufactured parts for features/characteristics.
Many vision systems include a camera for obtaining images of symbols or items to be imaged. A processor receives the images and extracts information which can then be used to perform one or more vision processes. In many applications the distance between a camera sensor and a symbol or item to be imaged may vary between uses. In these cases, in order to obtain useful images (i.e., images from which data required to complete machine vision processes can be extracted), an adjustable lens and auto-focus system is often provided. In these cases, when the system is activated to perform a vision process, the lens and auto-focus system automatically focus the lens so that a clear image of the symbol or item to be imaged is generated on the camera sensor. After the focusing process is completed, a clear image of the symbol or item to be imaged is obtained and is processed to complete the vision process.
One type of lens that can be used in a machine vision system is a liquid lens. Liquid lenses are constructed of one or more fluid of different refractive indexes, and can be varied by controlling the meniscus, or surface of the liquid. In one type of liquid lens, for example, two fluids are contained in a tube with transparent end caps. The first is an electrically conducting aqueous solution, and the second is a non-conducting oil. The interior of the tube is coated with a hydrophobic material, which causes the aqueous solution to form a hemispherical lens which can be adjusted by applying a DC voltage across the coating to decrease its water repellency in a process called electrowetting. Electrowetting adjusts the surface tension of the liquid changing the radius of curvature and adjusting the focal length of the lens.
Liquid lenses are extremely versatile, providing a highly variable focal length, without the need for moving parts. Liquid lenses, however, are subject to changes in focal length due to temperature changes and aging of the liquids in the lens. Temperature and aging can, for example, alter the refractive index of the liquids, or the dielectric constant.
Furthermore, when using a liquid lens in high speed applications, there may be insufficient time to capture a sufficient number of images of an object to optimize the focus for auto-focusing. For these applications, other methods must be used. The present disclosure addresses solutions to these issues.